Several lines of evidence suggest a close correlation between the renal handling of sodium and calcium under a variety of experimental conditions. However, direct evidence for a common transport pathway in any segment of the nephron has not been established. It is the purpose of these proposed studies to apply the recollection micropuncture technique to an examination of sodium and calcium reabsorption by the individual nephron of the rat. The specific objectives are: (1) To further delineate the mechanism of action of chlorothiazide. Thiazides are unique among diuretics in that they dissociate the normally close correlation between sodium and calcium excretion. An understanding of their mode of action will assist in discriminating sodium-dependent and sodium-independent calcium transport. The roles of parathyroid hormone (PTH) and extracellular fluid volume are to be examined in the acute and chronic effect of chlorothiazide. (2) To delineate the site(s) of action of PTH in the nephron and the degree to which this hormone affects sodium transport as well as calcium transport. (3) To delineate the site(s) of action of aldosterone in the nephron and the degree to which this hormone affects calcium as well as sodium transport. (4) To investigate the sensitivity of sodium and calcium reabsorption in the collecting duct to chlorothiazide, PTH and aldosterone. The experimental approach will include determination of urinary recoveries of labelled sodium and calcium injected into the late distal tubule.